The process of fabricating semiconductor devices requires the transfer of patterns present on a photomask onto a substrate where the semiconductor devices are to be formed. One method of transferring patterns is through photolithography. In a typical photolithography process, a layer of photoresist, a photosensitive material that changes its chemical composition as a result of light exposure, is deposited on a substrate. The photoresist is then selectively exposed in an exposure tool where a photomask containing patterns to be transferred is illuminated by light from a radiation source and an optical system in the exposure tool simultaneously projects all parts of the mask pattern onto the photoresist. After the exposure process, the photoresist is immersed in a developer so that the exposed parts of the photoresist remain or dissolve depending on the polarity of the photoresist used. The developing process leaves a photoresist pattern that is substantially similar to the mask pattern on the surface of the substrate.
An exposure tool used in a photolithography process typically consists of a radiation light source, an aperture, a condenser lens, a projection lens and a stage where the substrate is placed. The condenser and projection lens are typically optical systems comprising a plurality of lenses. During an exposure process, the photomask is placed between the condenser and projection lens. The radiation source, aperture and condenser lens form the illuminator portion of the exposure equipment which is used to illuminate the photomask. Lightwaves from the radiation source pass through openings in the aperture where it is thereafter collected by the condenser lens to form an illumination beam that is projected onto the photomask. When the illumination beam passes through the photomask, it is attenuated by the photomask. The attenuated illumination beam emerging through the photomask is known as an imaging beam and it is passed through the projection lens which form the optical portion of the exposure equipment. The projection lens is used to focus the mask pattern onto a substrate.
It has been observed that features with the same critical dimension on a photomask can be patterned onto a photoresist differently resulting in photoresist structures with different critical dimensions. FIG. 1 is a critical dimension (CD) versus pitch graph obtained by measuring the linewidth dimension of various photoresist structures formed from a series of line and space patterns on a photomask. The lines in the pattern have a common linewidth critical dimension of 45 nanometers (nm) on the photomask but are spaced from an adjacent line at varying pitches where each pitch is composed of a line and a space. In the example of FIG. 1, the structures are exposed with the numerical aperture of the projection lens set at 1.12 and using an off axis illumination aperture having an annular ring opening characterized by partial coherence factors, σin/σout=0.95/0.77.
Although the lines have a common linewidth dimension of 45 nm on the photomask, there is significant fluctuation in the photoresist critical dimension within the 150 nm to 200 nm pitch range. If the fluctuation in critical dimension is greater than the limit tolerated by device requirements the affected pitch range is referred to as the forbidden pitch. Forbidden pitches are undesirable as the critical dimension fluctuation prevents a circuit designer from being able to use the affected pitch range in his design.
A known method of mitigating the forbidden pitch problem is to selectively bias the CDs of the features on the photomask. For example, the dimensions of affected features can be scaled up or down so that the final linewidth dimensions formed in the photoresist are within the desired range. However, one of the problems with the biasing approach is that it is highly dependent on process parameters such as photoresist thickness, developer and exposure tools used.
Another method of mitigating the forbidden pitch problem is by adding scattering bar proximate the line features on a photomask. However, in tight pitch situation (for instance, in FIG. 1 the 190 to 220 nm pitch range), the space is too small for placement of scattering bars.
In view of the succeeding discussion, photolithography processes that mitigate the forbidden pitch effect are desirable.